Methylamines are generally prepared commercially by continuous reaction of methanol and ammonia in the presence of a dehydration catalyst such as silica-alumina. The reactants are typically combined in the vapor phase, at temperatures in the range of 300.degree. C. to 500.degree. C., and at elevated pressures. Trimethylamine is the principal component of the resulting product stream accompanied by lesser amounts of monomethylamine and dimethylamine. The methylamines are used in processes for pesticides, solvents and water treatment. From a commercial perspective, the most valued product of the reaction is dimethylamine in view of its widespread industrial use as a chemical intermediate (e.g., for the production of dimethylformamide). Thus, a major objective of those seeking to enhance the commercial efficiency of this process has been to improve overall yields of dimethylamine and monomethylamine, relative to trimethylamine. Among the approaches taken to meet this goal are recycling of trimethylamine, adjustment of the ratio of methanol to ammonia reactants and use of selected dehydrating or aminating catalyst species. Many patents and technical contributions are available because of the commercial importance of the process. A summary of some of the relevant art for methylamine synthesis using a variety of catalysts is disclosed in U.S. Pat. No. 5,344,989 (Corbin et al.).
Molecular sieves are important catalysts in the production of methylamines. Molecular sieves have proven useful in catalysis applications because some of them have appreciable acid activity with shape selective features that are not available in the compositionally equivalent amorphous catalyst. Molecular sieves are classified based on their elemental composition and are able to separate components of a mixture based on sizes and shapes of the components. The structure of molecular sieves are based on a three dimensional network of oxygen ions. Within the tetrahedra sites are cations. The AlO.sub.2.sup.- tetrahedra in the structure determine the framework charge which is balanced by cations that occupy non-framework positions.
Molecular sieves that have only Al.sup.+3 or Si.sup.+4 cations within their tetrahedra sites are advantageous in methylamine production. These aluminosilicate molecular sieves often are referred to as zeolites. Chabazite and rho zeolites have a common structural framework of eight-member rings of tetrahedral atoms that are often associated with catalytic selectivity for production of dimethylamine from methanol and ammonia. Chabazite zeolites, where the zeolite is derived from mineral sources and the silicon to aluminum ratios in said zeolites is less than about 2:1, as well as rho zeolites are known to be useful as catalysts for methylamines. See U.S. Pat. No. 5,569,785 (Kourtakis et al.) and references cited therein. The catalysts have geometric selectivity which permits the release of dimethylamine and monomethylamine from the zeolite pores. The use of natural H-exchanged and M-exchanged chabazites, where M is one or more alkali metal cations selected from the group consisting of Na, K, Rb and Cs, is disclosed in U.S. Pat. No. 4,737,592 (Abrams et al.).
U.S. Pat. No. 5,399,769 (Wilhelm et al.) discloses an improved methylamines process using synthetic chabazites as catalysts. Runs 3-5 in Table 5 show the methylamines distribution for different synthetic chabazites with a Si:Al ratio of about 2.5:1. The molar ratio of ammonia to methanol was 3.5:1. Such an excess of ammonia is known to decrease trimethylamine formation. The percentage of dimethylamine for each run was 26, 48.7 and 51.5, respectively.
What are needed and are of significant interest to the chemical industry are process improvements which suppress production of trimethylamine and optimize dimethylamine and monomethylamine yields.